Two genetic determinants mediated by a lactose metabolic plasmid from a strain of Streptococcus cremoris, encoding phospho-beta-galactosidase (p-gal) activity and a lactose-specific component of the PEP-dependent phosphotransferase system (lac-PTS), were cloned in E. coli. A cloned 3.5 kb PstI/AvaI fragment transformed mutants of Streptococcus sanguis deficient in lac-PTS and p-gal activities (lac83), or only p-gal (lac8), to a lactose positive phenotype. Mutant lac8, but not lac83, was similarly transformed by a 2.6 kb SstI/AvaI fragment. Southern blot hybridizations using cloned fragments as probes confirmed the presence of plasmid-specific sequences in the S. sanguis transformants, identified a fragment of S. sanguis chromosomal DNA into which the lactose-specific genes integrated, and established the presence of identical genetic determinants for lactose metabolism in plasmids from S. cremoris and Streptococcus lactis. Incompatibility studies were conducted with naturally-occurring deletion derivatives, and with in vitro constructed derivatives, of the broad host-range streptococcal plasmid, pAMbeta 1. The results obtained indicated that genes associated with incompatibility may be located a considerable distance from a previously defined replication region of pAMbeta 1, or that pAMbeta 1 may contain a second origin of replication. A series of transformation experiments with S. sanguis strains containing, or being transformed by, derivatives of the pAMalpha 1 delta 1 family of plasmids were conducted in a study of the regulation of plasmid replication. The results confirmed the requirement for a trans-acting component for the initiation of replication of the pAMalpha 1 delta 1-like replicons, and provided preliminary evidence for negative control of replication.